The present invention relates to an escapement arranged between a gear train and a sprung balance of a timekeeper.
The main escapements known in watchmaking will be recalled hereinafter. The escapement is placed between the gear train, namely the set of wheels and pinions which, from the barrel, transmits the driving force to the escapement wheel, and the regulating organ of most timekeepers. It is known that function of the escapement is to the oscillations of the regulating organ whether a balance or a pendulum. The advantages and drawbacks of known escapements and the consequences resulting from such drawbacks as regards the working of the timepiece will be described hereinafter.
The Swiss lever escapement, used for almost all watches, has an escapement wheel which co-operates with two pallet-stones the movement of whose fork is limited by banking-pins. The fork co-operates with an impulse-pin carried by a roller secured to the balance-staff. The system provides two pulses via the oscillation of the balance and is self-starting. Those skilled in the art know that in order to prevent the phenomenon of overbanking, where the impulse-pin can strike against the back of the horns of the fork, a safety device called a draw, which holds the fork against the banking-pin while the balance is turning through its free oscillation arc, is used. The effect of the draw is transformed into inclination of the locking-face of the pallet-stone, which causes the escapement wheel to recoil at the moment of release. This recoil has the drawback of braking the balance and thus consuming power. It should also be noted that the balance receives its pulses via the pallets and not directly via the escapement wheel.
The detent escapement, which is expensive and delicate, is used mainly in chronometry. It is formed of a wheel with pointed teeth which rest on a jewel called the locking stone. This jewel is carried by a spring called the detent, an extension of which extends within the range of action of the discharging-pallet carried by a safety roller secured to the balance. The pallet unlocks the wheel each time the balance oscillates. The tooth of the wheel leaves the locking-stone and another tooth, acting on the impulse-pallet carried by the impulse-roller, which is coaxial and secured to the small roller, gives an impulse to the balance. This system has the advantage of having a direct impulse and not having any recoil during unlocking. However, this system has the drawback of being prone to overbanking when the balance describes the free arc and if a shock is applied to the timekeeper during such period. The watch can then stop. It will also be mentioned that this escapement provides a single impulse to the balance during an oscillation, which slightly reduces the yield of the system. Finally and by design, this system is not self-starting which also constitutes a drawback.
The cylinder escapement includes an escape-wheel which co-operates with a cylinder on which is mounted the sprung balance. The cylinder is formed by a small polished steel tube with an opening into which the teeth of the wheel can successively penetrate. The two ends of the cylinder are closed by steel plugs carrying the cylinder pivot. This system is advantageous because of the direct impulses which it proposes. Moreover, it is self-starting and, by its very design, is fully secure against overbanking. However, the system has the major drawback that, during the free arc, the tip of the tooth of the wheel constantly rubs against the inner or outer shell of the cylinder, which consumes power.